The present invention relates to a billet heater and more particularly to a furnace for pre-heating a metal billet for a metal extrusion press.
In the extrusion of aluminum billets it is desirable to pre-heat and thus soften the metal to some extend. Prior art devices for carrying out this step have relatively poor thermal efficiency. This is due to the design of the interior contour of the chamber and the method of thermal energy transfer which is limited by a short contact time between the billet and the combustion gases and by the low velocity of the hot gases. In such prior art furnaces the burners are situated within the furnace chamber on both sides of the billet and direct their flames non-tangentially against the underside of the billet. The flames pass only partially around the billet before being exhausted to the furnace exterior. Thus, the flames, although of high temperature, have a low mass velocity and only contact the billet for a short period of time. Thermal efficiency, however, is a function of both the mass velocity of the combusted gases, the length of time the gases are in contact with the billet, and the gas temperature.
Another problem of some prior art aluminum billet heaters is that the temperature maintained within the furnace is controlled by means of a thermocouple which is inserted through the side of the furnace and which contacts the billet as it moves through the length of the furnace. This thermocouple probe develops an aluminum oxide coating from the billet which acts as a thermal insulator and miscalibrates the probe. Still another problem of some prior art billet furnaces is that they are not readily adaptible between oil and gas fuels, but must be specially converted for each one.